Currently, many isolating type power supplies such as cell phone charger and high-power LED driver are often required to have a capability of outputting a constant current to meet the requirement of some applications. In addition, to relieve the pressure of power pollution and satisfy the Harmonic standards such as IEEE555-2 and IEC1000-3-2 set by International Electrotechnical Commission, the above isolating type power supplies are also required to be imbued with power factor correction (PFC) functionality FIG. 1 illustrates a commonly adopted solution for single-stage power factor correction. By detecting the output current at the secondary side of the transformer, the output current is fed back via an optical coupler to a primary-side PFC control circuit after the output current has gone through a constant-current control at the secondary side. According to the prior art solution as shown in FIG. 1, circuit complexity is increased due to the existence of a circuit for sampling the current at the secondary side and the optical coupler. Further, since the optical coupler has aging problem, the stability and the lifespan of the circuit are both affected to some extent.
A control solution with both primary-side constant current control function and power factor correction function is directed to solving the above problems. That is, there is no need to sample the secondary-side current and no need for the optical coupler. Information relating to the output current is obtained directly at the primary side of the isolating type transformer. The information is controlled to realize constant current output and high power factor, as illustrated in FIG. 2. Currently, some controller chips for realizing constant current output and PFC function are commercially available. The most important two indicators evaluating the above control solution are the high power factor of the input current and the accuracy of the constant output current. Usually, since primary-side control is adopted, the constant output current is less accurate than the secondary-side controlled constant current.
Currently, a prior art solution for outputting constant current is simulating the secondary-side current at the primary side. That is, the secondary-side output current is simulated or the secondary-side average output current is calculated and then constant current is controlled at the primary side. As shown in FIG. 3, by sampling and holding the primary-side current “ipri”, the primary-side current peak and the corresponding secondary-side current peak are obtained, where “ipri” denotes the primary-side current signal, “Vcontrol” denotes the sampled signal, “isample” denotes an output signal of a sample and hold module, and “iemu” denotes an output signal of a secondary-side current simulation module. However, in the actual circuit, since there exists a time delay during the switch between sample and hold performed by the sample and hold module, error may occur in the sampled primary-side current peak which in turn results in the difference between the simulated secondary-side current “iemu” and the actual value, as shown in FIG. 4. Moreover, the difference may vary with the input voltage and the magnetizing inductance of the transformer. However, such difference may not be compensated easily. Thus, the output constant current may vary with the input voltage and the magnetizing inductance of the transformer, resulting in a low accuracy of the output constant current.
Another prior art method for outputting constant current is a constant power method, as illustrated in FIG. 5. A half-wave rectified signal Vin is obtained after an input AC signal is rectified. An effective value of the input AC signal (i.e., an input voltage feed-forward signal Vff) is obtained after the half-wave rectified signal Vin passes through a voltage feed-forward module. Also, a waveform signal Mc is obtained after the input voltage feed-forward signal Vff passes through the waveform correction module K1. The waveform signal is expressed as Iac=kxVin, where K is a coefficient. In the circuit for outputting constant current, Vea is a controllable constant. The multiplexer multiplies the waveform signal Iac, the input voltage feed-forward signal Vff and the controllable constant Vea together to get a current reference signal:
      I    ref    =                              I          ac                ×                  V          ea                            V        ff        2              =                  k        ×                  V          in                ×                  V          ea                            V        ff        2            
As such, the conductance current can be controlled to be in line with the current reference signal in order to realize PFC function. As can be seen, the multiplier makes the square of the input voltage feed-forward signal as the denominator. When Vea is constant, the input power is irrelevant with the input voltage, that is, constant power can be controlled. The above-described method for obtaining a current reference by utilizing a multiplier to offset the influence of the input voltage can be essentially referred to as voltage feed-forward control. However, in the presence of a phase-controlled dimmer, the input AC signal may be incomplete when the dimming phase is different. The rectified input AC signal may no longer be a complete half wave. Therefore, the input voltage feed-forward signal Vff contains a phase-cut signal. The voltage feed-forward control may result in a sharp increase in the Iref as the cutting phase increases. The input power also increases dramatically accordingly. Therefore, the foregoing method is not applicable to the phase-controlled dimming.